In certain applications it is desired to render a sound over headphones to a listener such that the sound is perceived as coming from a specific position in space outside of the listening person's head. A typical application is Virtual Reality, where the listener would wear a Virtual Reality head-set that presents a visual image together with headphones that complement the experience with audio. Sometimes Virtual Reality is also referred to as Augmented Reality, AR, or X Reality, XR, where X stands for “unknown”.
A head-tracking solution is normally used that senses which general direction a person is facing in relation to a sound source. What the person sees and hears is consequently updated in real time so that the person can look in different directions in the virtual world and get an experience of being there.
When the listener turns, the relative direction to sound sources in the virtual world changes. Head-tracking information is thus needed so that sound sources can be rendered in their correct place in the virtual world regardless of which direction the listener is facing.
By way of example, a common method used to render binaural virtual audio sources is based on convolving an audio source signal with a binaural HRTF response. The HRTF (Head Related Transfer Function) is comprised of one transfer function for each ear which has encoded in it the necessary information that the brain needs to sense the direction of the sound. Described simply, a HRTF response can be measured by placing microphones in the ears of a person, then placing a sound source at a specific angle and distance to the person and then measuring the transfer function (or equivalently, the impulse response) from the sound source to each of the listener's ears. The angle of the source is specified by the 3D polar angle in relation to the head, with origo in the center of the head.
By measuring HRTFs for a large number of directions to a person, a database of HRTFs can be constructed and this enables the rendering of virtual sound sources at many different locations as well as moving sound sources. The way that an HRTF database is normally measured is by having the person that the measurements are taken on sit still and face in a very precise direction during the measurements while the position of the sound source is changed for each measurement. Since the person sits still while the position of the sound source is changed, the head and torso of the person are kept in the same constant relation during the measurements. More information about the current state of the art in binaural technology can be found in reference [1].